Optical communication systems encode digital information onto an optical carrier signal by modulating various signal characteristics. A simple form of encoding, for instance, modulates the amplitude of the optical carrier signal by switching it between an on state and an off state. An example of this simple form of encoding is on-off keying (OOK). More advanced forms of encoding may modulate other signal characteristics in addition to amplitude, such as phase, and polarization. Examples of these more advanced forms of encoding include binary phase-shift keying (BPSK), which encodes digital information using two symbols, and quadrature phase-shift keying (QPSK), which encodes digital information using four symbols.
In an optical communication system using advanced encoding, a receiver typically decodes the optical carrier signal using a combination of one or more couplers, phaseshifters or delay lines, photodetectors (PDs), and analog to digital converters (ADCs). For instance, the receiver may use an optical phase-quadrature (IQ) hybrid in combination with balanced or unbalanced photodiodes to down-convert the optical carrier signal to an electrical baseband frequency range, and it may use a pair of ADCs to digitize the resulting electrical signals for subsequent processing.
To measure and subsequently decode optical signals that have been encoded at a relatively high rate, it is generally necessary to use ADCs, PDs, and other components that have commensurately high bandwidth. Unfortunately, however, available ADCs, PDs, and other components, may be too slow or too expensive to perform decoding at the relatively high rates required by some optical communication systems. For instance, even though the channel bandwidth of an optical communication system may be only a small fraction of the carrier frequency, it may nevertheless reach several tens of gigahertz (GHz), which can challenge the bandwidth of even the fastest and most expensive ADCs and PDs on the market.
In view of these and other shortcomings of conventional optical measurement or communication systems, there is a general need for new approaches to optical measurement and decoding that overcome the restrictions imposed by existing component technologies.